The present invention is generally related to ceramic filter and more particularly to an improved ceramic filter having an integral phase shifting network especially adapted for use in antenna duplexers.
Communications equipment that includes both a transmitter and receiver using a common antenna usually requires a network to route transmitted and received signals properly. Received signals coming from the antenna must be directed to the receiver without significant loss to the transmitter. Similarly, transmitted signals from the transmitter must be directed to the antenna without significant loss to the receiver.
In the past, filtering networks such as that described in U.S. Pat. No. 3,728,731 have been used to route the signal appropriately. When the selected filters had highly reactive out-of-passband impedances, transmission lines were often used to connect transmit and receive filters to the antenna (see foe example, U.S. Pat. No. 4,692,726). The lengths of those lines were chosen so that at the junction of the transmit and receive paths, the transmit path would appear as an open circuit to signals in the receive band, and the receive path would appear as an open circuit to signals in the transmit band.
Problems with using this method will arise when the out-of-passband impedance of one of the filters is capacitive at the passband frequencies of the other filter. This situation will require a transmission line for duplexing that is one quarter to one half wavelength long. This rather long transmission line results in two detrimental effects. First, the loss of this transmission line will add to the passband loss of the filter it is connected to, thereby increasing the path loss to the antenna. Secondly, the loss of this transmission line will reduce the out-of-band impedance seen at the junction of the transmit and receive paths, thereby reducing the effectiveness of the duplexing network. In addition to these problems, a long transmission line requires an excessive amount of space to implement, and tuning of the length of line to compensate for unit-to-unit variations in the line itself or the filters out-of-band impedance is difficult.